The structure, physical properties, purity, and chemical inertness of pyrolytic boron nitride (PBN) make it an attractive container material for elemental purification, compounding, and growth of semi-conductor crystals. Examples include containers for liquid-encapsulated Czochralski (LEC) and vertical gradient freeze (VGF) growth of GaAs and other III-V and II-VI compound single crystals, and source containers for deposition of metals and dopants at high temperatures and ultra-high vacuum by molecular beam epitaxy (MBE). Recently, pyrolytic boron nitride has been used as a container for growth of GaAs crystals by a liquid encapsulated vertical zone melting process. GaAs crystals with extremely low carbon content have been produced in liquid-encapsulated Czochralski furnaces where the graphite furnace parts were coated with pyrolytic boron nitride.
Pyrolytic boron nitride can be produced by various methods such as the method disclosed in U.S. Pat No. 3,152,006, in which pyrolytic boron nitride is produced by the vapor-phase reaction of ammonia and boron halides, such as boron trichloride. By depositing the boron nitride produced in this manner upon a suitable mandrel, such as a graphite mandrel, a wide assortment of shapes can be produced. Boats of pyrolytic boron nitride have been produced in this manner with smooth surfaces that may be wetted by a molten element or compound. As used herein, wetting shall mean a physical surface reaction between a molten material and the surface of a boat which results in sticking and/or adhesion of the molten material on the surface. Wetting results in internal stress during solidification of the molten material (for example, an element or compound) and as a consequence, imperfect product may be produced. For example, when producing single crystals in smooth surface boats, the wetting of the walls by the molten material can cause internal stress during solidification which can result in imperfect crystal formation, i.e., polycrystal or twinning.
Most gallium arsenide single crystal is grown in quartz horizontal boats. This material is characterized by its low electrical resistance and high degree of crystal perfection. The low electrical resistance is generally a result of silicon contamination (autodoping) from the quartz boat. The level of autodoping is generally non-uniform along the boat length with the lowest level at the seed end. This autodoping of silicon from the quartz boat limits the crystal produced to applications not requiring high electrical resistance, i.e., light emitting-diodes. It is disclosed in the literature that twinning can be reduced by sandblasting the surface of the cavity of a quartz boat in which the single crystal is grown.
Elimination of silicon autodoping would improve electrical properties uniformity of these crystals and expand their use to applications requiring a high electrical resistance, i.e., integrated circuits. Pyrolytic boron nitride boats have been used to produce undoped high resistance gallium arsenide crystals in which the silicon contamination was effectively eliminated but, however, the formation of single crystal growth was hindered because of a surface reaction (wetting) between the gallium arsenide melt and the surface of the pyrolytic boron nitride boat. This interface problem cannot be overcome by sandblasting the pyrolytic boron nitride boat because of the laminar nature of the pyrolytic boron nitride structure. The thickness of the pyrolytic boron nitride lamina is generally too thin (about 0.5 mil thick) to allow sufficient surface roughness to occur before the entire lamina is removed and a fresh, smooth lamina exposed.
It is an object of the present invention to provide a process for producing a pyrolytic boron nitride boat having a cavity with a nodular surface that is ideally suited for use in the growth of single crystal materials.
It is another object of the present invention to provide a process for producing a pyrolytic boron nitride boat having a cavity with a nodular surface that is ideally suited for the growth of gallium arsenide.
It is another object of the present invention to provide a pyrolytic boron nitride boat having a cavity with a nodular surface that is ideally suited for the growth of gallium arsenide.
The foregoing and additional objects will become fully apparent from the following description and the accompanying drawings.